We propose to examine visual transduction in rods and cones of humans and Macaca fascicularis, a monkey with photoreceptors similar to those of man. We will record the photocurrents of single cells to determine their response properties, electrical noise, and spectral sensitivities. This experimental information will be used to develop a quantitative picture of the way in which primate photoreceptors encode visual stimuli. We intend to approach the following questions: 1) What are the kinetics of the photocurrents of rods and the three kinds of cone? 2) How is intensity information represented? 3) How much, and over what range of intensities do background lights desensitize? 4) What electrical noise is present in darkness and after exposure to bright (bleaching) light? 5) What is the spectral sensitivity of the rods and the three kinds of cone throughout the visible region of the spectrum? 6) Do the functional parameters of the transduction mechanism vary systematically with longitudinal position in the outer segments of the rods and cones? 7) Are primate photoreceptors electrically coupled?